1. Field of Invention
The present invention relates generally to a control method and system for a device to be connected to the internal combustion engine of a vehicle, especially an automobile, which supplies a reformed fuel to the internal combustion engine.
2. Description of the Prior Art
A partial oxidation method has been available for reforming a liquid fuel, but the reaction in this method being an exothermic one, causes heavy loss of the energy in the liquid fuel, resulting in an increased consumption of fuel. A gas yielded through reformation of fuel, hereafter called "reformed gas," unlike a liquid fuel, such as gasoline, possesses a very wide range of combustion and a high octane value. Thus, use of a reformed gas as a fuel, with appropriate selection of an internal combustion engine and appropriate combination with a car-mounted fuel reformer, hereafter called "reformer," will bring about an increased overall efficiency of energy utilization and a saving of fuel consumption. The main component of hydrocarbons in the reformed gas is methane (CH.sub.4). Since methane (CH.sub.4) is poor in photochemical reactivity and stable in combustability, when used at the same rate of air excess as gasoline, methane will lower the content of hydrocarbons in the exhaust gas. Moreover, when the reformed gas is burned as a lean mixture, the generation of harmful components in the exhaust gas, such as nitrogen oxides and carbon monoxide, can be substantially inhibited.
Namely, operation with a combination of a reformer and an internal combustion engine will be far more advantageous than operation with a direct supply of liquid fuel, like gasoline or gaseous low-class hydrocarbons, to the internal combustion engine, but an appropriate control is needed for this purpose.
Now the reformer is to be considered. Partial oxidation of n-heptane, as an example of liquid fuel, can be expressed by: EQU C.sub.7 H.sub.16 + 3.50.sub.2 - 7CO + 8H.sub.2
(calorific value: 146 Kcal/mol)
The air/fuel ratio in this reaction is about 4.8. It is known that the air/fuel ratio has to be slightly higher than the above value for the purpose of maximizing hydrogen generation, but this is restricted by the material quality of the reformer, the lower limit of the optimum reaction temperature, and segregation of carbon and thermal efficiency, while for this purpose of adapting the reformed gas to the internal combustion engine, the calorific value per unit volume and the octane value of the reformed gas have to be limited. Only when all these conditions are satisfied can lean-burning, which characterizes the reformed gas, take place efficiently and effectively. Thus, to give full play to the merit of the reformed gas, the operating condition of the reformer has to be swiftly and properly changed, corresponding to changes in the running conditions of the internal combustion engine, such as, for example, the rpm thereof.
Generally, the reaction temperature, the air/fuel ratio and the reaction volume (material supply rate) in the reformer have influence on the quality and quantity of the reformed gas. For instance, the driver, when he wants an increase in the output of the engine, has to increase the air inflow to the reformer and, depending on the air inflow, also increase the supply of liquid fuel. Meanwhile, for the sake of increased output of the engine, it is desirable not only that the supply of the reformed gas to the engine be increased, but also that the calorific value of the reformed gas per unit volume be also increased. Thus, it is necessary to increase the rate of air inflow to the reformer and decrease the air/fuel ratio. In this way, not only the calorific value per unit volume of the reformed gas generated in the reformer can be increased, but also excessive rise in the reformer temperature can be prevented.
On the contrary, the driver, when he wants a decrease in the engine output, has to decrease the rate of air inflow to the reformer and at the same time decrease the supply of liquid fuel to the reformer. Thereby, since a decrease in the reaction volume is liable to lower the catalyst bed temperature, it is desirable that the air/fuel ratio be increased to prevent a temperature drop.
Thus, in a practical run, the reformer is required to perform an extremely wide range of reactions and, understandably, the air/fuel ratio has to be changed depending on the reaction volume.
Next, the control of gas drawn into the engine is to be considered. The air/fuel ratio of the gas mixture to be burned in the engine is desirably changed to correspond to the running conditions. The internal combustion engine burns a reformed gas generated in a reformer or a liquid fuel which does not pass the reformer or a mixture of these two. When a reformed gas generated in the reformer or a mixture of such with a conventional liquid fuel is employed, the burning range is greatly enlarged, thus enabling lean-burning. Thus, the contents of harmful components, especially of nitrogen oxides, in the exhaust gas can be substantially decreased, while, at the same time, the fuel consumption per unit output can be decreased, as is well-known. However, gaseous fuel, like the reformed gas, is characteristically poor in drawing or sucking efficiency and, accordingly, the maximum output drops unavoidably in the practical range of air/fuel ratios. For this reason, the driver, when he wants a high output, reduces the air/fuel ratio in the gas mixture to be drawn into the engine; increases the air/fuel ratio in the practical working range; and, in a very low speed range or in an idling condition, he increases the air/fuel ratio further or decreases the air/fuel ratio with the throttle valve closed. Thus, control of the ratio of air and fuel drawn into the engine is far more complicated than in the reformer.
For this reason, it is necessary to develop a method and system by which the driver can swiftly and properly effect the aforementioned complicated control by a simple manipulation.